I've been doing a lot of work on the Cisco 509G and the 525G and their use with the Elastix / Asterisk System.  The result?  Awesome.  The phones work great and they have great indicators for the users.  Audio quality is good and the users seem to really like them.  They are harder to setup than some other phones - notably the Aastra telephones, but I think it is worth it in the long run.  We've got a bunch in production now and they are humming away!

On another note - I added a training video for users of the phones on Asterisk / Elastix.  Click here to get to the training video.